[IMP] component: disable mutations in render phase

closes #48

src/component.ts

@@ -227,7 +227,6 @@ export class Component<
     }
     this._patch(vnode);
     target.appendChild(this.el!);
-    this._observeState();
 
     if (document.body.contains(target)) {
       this._visitSubTree(w => {
@@ -357,6 +356,7 @@ export class Component<
           true
         );
       }
+      this._observeState();
       return this._render();
     });
     return this.__owl__.renderPromise;
@@ -366,11 +366,13 @@ export class Component<
     this.__owl__.renderId++;
     const promises: Promise<void>[] = [];
     const template = this.inlineTemplate || this.template;
+    this.__owl__.observer.allowMutations = false;
     let vnode = this.env.qweb.render(template, this, {
       promises,
       handlers: this.__owl__.boundHandlers,
       forceUpdate: force
     });
+    this.__owl__.observer.allowMutations = true;
 
     // this part is critical for the patching process to be done correctly. The
     // tricky part is that a child widget can be rerendered on its own, which

tests/component.test.ts

@@ -1426,4 +1426,27 @@ describe("widget and observable state", () => {
     await nextMicroTick();
     expect(fixture.innerHTML).toBe("<div>beer</div>");
   });
+
+  test("subwidgets cannot change observable state received from parent", async () => {
+    expect.assertions(1);
+    class Parent extends Widget {
+      state = { obj: { coffee: 1 } };
+      widgets = { Child };
+      inlineTemplate = `<div><t t-widget="Child" t-props="state.obj"/></div>`;
+    }
+    class Child extends Widget {
+      constructor(parent, props) {
+        super(parent, props);
+        props.coffee = 2;
+      }
+    }
+    const parent = new Parent(env);
+    try {
+      await parent.mount(fixture);
+    } catch (e) {
+      expect(e.message).toBe(
+        'Observed state cannot be changed here! (key: "coffee", val: "2")'
+      );
+    }
+  });
 });